Composites are made by exposing fiber reinforced polymers to pressure and/or heat. Typically, fibers are first impregnated with polymer to form a prepreg and the prepreg is exposed to elevated temperatures and/or pressures to cure the composite. Alternatively, polymer can be impregnated into fiber layups under pressure and then cured in a resin transfer molding process Typically, a mold surrounds the fiber layup and resin is pumped into the mold whereupon pressure and sometimes heat is applied to "transfer" the polymer into the fiber layup. Typically, heat and pressure are applied to cure the polymer impregnated fibers. However, resin transfer molding can have problems For example, because the fiber layup acts as a filter, the transfer of the resin is inhibited, resulting in uneven resin distribution.
A variety of pressure transfer means are used to apply pressure to a reservoir of polymer in order to transfer the polymer into the mold cavity including pistons, pumps, etc One molding method is described in commonly assigned European Patent Application No. 87630010.4 entitled "Molding Method and Apparatus Using a Solid, Flowable, Polymer Medium" to Kromrey. An article is molded by contacting it with a solid polymer medium, such as an especially low strength unfilled silicone rubber which is solid and able to flow readily. Thermal expansion of the medium or mechanical force is used to create molding pressure and thereby provides a substantially uniform pressure on the article precursor. Various temperature and pressure cycles can be attained; constant high pressures can be maintained on the article precursor during cooldown, optionally aided by flowing of medium to and from a vessel in which the article precursor is being molded. The method is particularly adapted to molding filler or fiber reinforced thermosetting polymer composite articles.
Although there are a wide variety of molding techniques (e.g., resin transfer) there is a continual search for molding methods that produce denser, more uniform parts.